Engineers break power and distance barriers for fiber optic communication

“Crosstalk between communication channels within a fibre optic cable obeys fixed physical laws: it’s not random”.

Scientists from the University of California in San Diego wrote on the journal Science earlier this week that it is possible to “pre-distort” data transmitted via laser beams, allowing it to be crunched and translated even over considerable distances.

With the demand for data increasing at a staggering rate amid the rising popularity of streaming music and movies, among other technologies, fiber optic cables are also under a rising amount of pressure.

In lab experiments, the researchers successfully deciphered information after it travelled a record-breaking 12,000 kilometres through fibre optic cables with standard amplifiers and no repeaters. This ‘going without the repeaters’ was the highlight of the study. These regenerators are effectively supercomputers and must be applied to each channel in the transmission. This electronic regeneration in modern transmission dictates the cost and, in particular, prevents the construction of a transparent optical network. The new approach will eliminate this need, leading to cheaper, more efficient information transmission, as well as making the entire economy of the network infrastructure a target for new models and market participants. Instead, in August past year, Alic and colleagues proposed using a frequency comb – in which one laser produces a series of pulses with equally spaced frequencies that act as a “ruler” – and transmitting each channel using a different tooth of the comb. If the fundamental laser frequency changes, all of the teeth move in step, so the relative frequency doesn’t change and the nonlinear interaction is unaffected. “In this study, we present a method for leveraging the crosstalk to remove the power barrier for optical fiber”, explained Stojan Radic, a professor in the Department of Electrical and Computer Engineering at UC San Diego and the senior author on the Science paper. These experiments on photonics were conducted at the Qualcomm Institute at the University of California, by researchers from the Photonics Systems Group, also led by Radic.

While there are many benefits of using fibre-optic fibres to transmit data, one of the most significant is that photons don’t significantly interact with one another, so multiple optical signals can travel down the same line.

The researchers liken their solution to that of a conductor who tunes multiple instruments in an orchestra to the same pitch at the beginning of a concert. This approach compensates in advance for the crosstalk that occurs between the multiple communication channels within the same optical fiber. Conversely, when the channels were transmitted via the teeth of a frequency comb, the researchers could reduce crosstalk far more effectively. The frequency comb ensured that the system didn’t accumulate the random distortions that make it impossible to reassemble the original content at the receiver. When the three channels were transmitted by three separate lasers, the researchers found that, despite their attempts to compensate for it in advance, there remained significant crosstalk between the signals, and as a result, signal clarity declined if they put more than 200 ?W of power into each signal.

As a result, the researchers theorized past year that a signal could be pre-conditioned in such a way that when the signal eventually degraded, it would do so in a way that was predictable, allowing them to accurately reconstruct the original signal by filling in the gaps caused by the distortion. With the frequency comb, the information can be unscrambled and fully restored at the receiving end of the optical fiber.


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